Saturday, 27 November 2010

Exception Safe By Design

I’ve recently gone back to my simple database query tool to add a few essential usability features and like any legacy codebase there were a couple of surprises in store. Now, I’m not making excuses for myself, but most of the code is the best part of 10 years old and like any journeyman I’ve learnt a lot since that was written. Also the recent episodes of Matthew Wilson’s Quality Matters column in Overload (one of the ACCU journals) focused on exception handing and so these two examples became even more prominent.

Resource Management

One of the new features I added was to properly support non-query type statements, i.e. INSERT/DELETE (it was originally written to be a query tool but it’s been useful to run ad-hoc SQL as well). This involved me digging into the ODBC facade and eventually I came across this piece of code:-

Now clearly you’ll have to put aside your disgust of Hungarian Notation for a moment and focus on the real issue which is the shear verbosity of the code. It’s also wrong. On the one hand I’ve been a good boy and only caught what I can knowingly recover from (a specific CODBCException type exception) but I’ve also missed the point that any other exception type will cause a memory leak. Clearly I should have used catch(…) because the exception type is irrelevant in this case.

Only I shouldn’t, because the try/catch block is entirely unnecessary. Although now I’m firmly in the camp that believes RAII is probably the single most important idiom within C++, I obviously wasn’t then. I try really hard to write code that ensures there are no memory leaks and I use tools like the Debug CRT Heap in Visual C++ and BoundsChecker to point out my mistakes. And yet this piece of code highlights how hard it is to do that, especially in the face of exceptions; if you don’t have 100% code coverage in your testing it’s just too easy to miss something. What is all the more ridiculous though is that the correct solution is less work anyway:-

Assuming I can make no changes to the method signature this ticks the box of being exception safe by design. The Core::UniquePtr smart pointer class is analogous to boost::scoped_ptr which ensures single ownership until it’s given away at the end. But there’s still a bad code smell which is that the method returns a bald pointer so the callee could still leak it. What we should return instead is the smart pointer object so that we explicitly transfer ownership to the callee:-

One unpleasant taste in the mouth is that because of the change in scope we cannot just return a Core::UniquePtr because that would require temporarily copying the object[*] so instead we use a smart pointer that allows shared ownership – Core::SharedPtr. Funnily enough this transfer of ownership from caller to callee is actually right up std::auto_ptr’s street and for the knowledgeable this would be preferable from a performance perspective. But sadly you end up worrying about the caller and whether they know the pitfalls of auto_ptr and so perhaps you go back to the safer option.

Side Effects

Another feature I wanted to added was to avoid requesting credentials when the connection doesn’t need them and this put me right into the client method responsible for opening a new database connection:-

This piece of code I found puzzling because I couldn’t obviously see why it called OnDBDisconnect() both at the start of the method and also in the exception handler as the connection will either open successfully or throw, in which case we had already cleaned up earlier. So I looked at what OnDBDisconnect() did to see why it might be necessary:-

There are side-effects that occur between the point when the previous connection is closed and when we attempt to open a new one. A quick scoot back through the version control repository[+] to see what changes have been made to this method and I see that it was checked in as a fix for another problem after failing to open a connection. Clearly my mindset was “fix the broken exception handling” instead of “fix the poorly written method”. The first rule of writing exception safe code is to perform all side-effects off to one side and then use exception safe mechanisms like std::swap()[^] to “commit” the changes.

So, for my example code above we need to introduce some temporaries to hold the unstable state, perform the Open and then persist the new state:-

Taking a cue from the preceding section about resource management I have used my Core::SharedPtr[$] smart pointer type as the connection will live on after this method returns. The catch handler remains but only to report the error message to the user, no cleanup now needs to be performed.

But we can do even better. If we take a closer look at what happens as a result of using swap() we see that the call to OnDBDisconnect() may actually be superfluous. In fact it may be better not to do it all. The call to swap() effectively means that the new connection will live on as App.m_oConnection and the old one will get destroyed at the end of the scope (the try/catch block). Now if we look at what OnDBDisconnect() does:-

It closes the existing connection and resets m_pCurrConn (these both now happen automatically near the end of Connect()) and it resets the UI. If we look at the tail end of the Connect() method we see this:-

UpdateScriptsMenu(); UpdateUI();

What this all means is that if an exception is thrown then the user loses their current connection as well. If we perform all the work off to the side and don’t modify any state (i.e. don’t call OnDBDisconnect()) up front then the user at least gets to keep their current connection as a consolation prize. If the new connection does work then we just save the new state and let the old one disappear at the end of the try/catch block. Is that just being clever or an attempt to write the simplest code possible? A topic for another day…

[*] I need to go back and read up on the new std::unique_ptr type in C++0X as I’m sure this was designed for this kind of scenario – and also the std::vector<std::auto_ptr<T>> gotcha.

[#] Using an exception to signal failure when opening a database connection falls right into the “Using Exceptions For Flow Control” category in Matthew Wilsons Quality Matters column and would almost certainly fall into the real of “Not Exceptional” for Kernighan & Pike too. In this case I’m inclined to agree and will be adding a TryOpen() method at the next opportunity.

[^] It would be great if std::swap() had a ‘nothrow’ guarantee as that would make it excellent for this idiom. But I’m not sure it is guaranteed by the standard - is it just a convention like not throwing exceptions from destructors? My Google skills found lots of chatter but no quote citing chapter & verse.

[$] Naturally you wouldn’t use the full type name everywhere but use a typedef instead. Given how prevalent the use of the SharedPtr type is (especially with types stored in containers) I tend to add a typedef right after a new entity class declaration for the smart pointer wrapper, e.g. class Xxx { . . . }; typedef Core::SharedPtr<Xxx> XxxPtr;

1 comment:

I just checked the latest draft of the next standard, and it does not seem that std::swap has a 'nothrow' guarantee...C.f. section 20.3.2 [utility.swap] in http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2010/n3126.pdf